As this is an interview question, a lay man kind of answer can be -- Using ISPF 3.14, search the JCL in which this DSN is created/used, pick the DD name from there and search that DD name in the COBOL source library. You'll get the program using that DD name and so the layout.

For the sake of discussion, let's assume there is only one copybook per DSN -- though what Enrico has said is the actual and practical situation. This question is kind of BS for an interview.

Once you find the dsn in the JCL (it will probable appear in more than 1 JOB or step), find the associated PGM that uses the DSN. Then look at the associated FD for the copybook. If not there, it's probably in WS. Check the READ/WRITE stmts for FROM/INTO.

Thanks Jack -- that's rather straight forward. Not sure why did I not think to find the associated PGM that uses the DSN, ..mmmm... why in the heaven I thought to search in the "source-base-library" . . . ... that's ofcourse time-consuming...